Embodiments of the invention relate generally to MR coils and, more particularly, to an apparatus and method of decoupling MR coils.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Accelerated MR imaging techniques have been developed to expedite MR data acquisition thereby reducing scan time and increasing subject throughput. One known accelerated MR imaging technique is parallel imaging whereby a phased coil array samples an imaging volume. Generally, scan time reduction is achieved by under-sampling k-space and recording images simultaneously from multiple imaging or receive coils. Under-sampling generally reduces the data acquisition time and the use of multiple receive coils, such as a phased coil array, reduces wraparound caused by under-sampling. In this manner, scan time is reduced by increasing the distance of sampling positions in k-space. If an image space is under-sampled in the phase encoding direction, for example, by a factor of two, then it will take half the time to acquire the image. In this regard, every pixel in the image will represent data from two spatial points.
In a phased coil array, a plurality of coils may be arranged in a row direction and in a column direction. Typically, the coils in the column direction are overlapped to reduce a mutual inductance between the coils in the column direction. In the row direction, however, the coils are often underlapped to provide greater spatial coverage in the row direction. This underlapping typically causes neighboring elements to be coupled via mutual inductance. To counter this effect, a transformer may be introduced between neighboring coils to cancel the mutual inductance. This transformer typically has two coupled solenoids. The coupling constant between the neighboring coils can typically be adjusted via bending one or both of the primary and secondary windings of the transformer or via adjustment of a metal setscrew on the transformer. Alternatively, a trim capacitor may be coupled between the primary and secondary windings, and the trim capacitor may be adjusted to vary the coupling constant. The transformer, however, increases a build-up height and adds cost and weight to the phased coil array.
It would therefore be desirable to have an apparatus and method capable of tuning neighboring coils while reducing the build-up height, cost, and weight of a phased coil array.